<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        /***
         *  ! : 将表达式的布尔值取反
         *  
         *  快速获得表达式的布尔值：
         *  Boolean(xxx)   !!xxx
         */
        console.log(!8); // !Boolean(8)

        console.log(Boolean(8));
        console.log(!!8);
    </script>
</body>
</html>